Problem: Find the distance between the point ${(-2, -8)}$ and the line $\enspace {y = -x }\thinspace$. ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$ ${1}$ ${2}$ ${3}$ ${4}$ ${5}$ ${6}$ ${7}$ ${8}$ ${9}$ ${\llap{-}2}$ ${\llap{-}3}$ ${\llap{-}4}$ ${\llap{-}5}$ ${\llap{-}6}$ ${\llap{-}7}$ ${\llap{-}8}$ ${\llap{-}9}$
Solution: First, find the equation of the perpendicular line that passes through ${(-2, -8)}$ The slope of the blue line is ${-1}$ , and its negative reciprocal is ${1}$ Thus, the equation of our perpendicular line will be of the form $\enspace {y = x + b}\thinspace$ We can plug our point, ${(-2, -8)}$ , into this equation to solve for ${b}$ , the y-intercept. $-8 = -2 + {b}$ $-8 + 2 = {b} = -6$ The equation of the perpendicular line is $\enspace {y = x - 6}\thinspace$ We can see from the graph (or by setting the equations equal to one another) that the two lines intersect at the point ${(3, -3)}$ . Thus, the distance we're looking for is the distance between the two red points. The distance formula tells us that the distance between two points is equal to: $\sqrt{( x_{1} - x_{2} )^2 + ( y_{1} - y_{2} )^2}$ Plugging in our points ${(-2, -8)}$ and ${(3, -3)}$ gives us: $\sqrt{( {-2} - {3} )^2 + ( {-8} - {-3} )^2}$ $= \sqrt{( -5 )^2 + ( -5 )^2} = \sqrt{50} = 5\sqrt{2}$ The distance between the point ${(-2, -8)}$ and the line $\thinspace {y = -x }\enspace$ is $\thinspace5\sqrt{2}$.